1. Field of the Invention
This invention relates to interferometric fiber optic sensor systems, and in particular relates to interferometric fiber optic sensor systems that are self-referencing. In a particularly preferred embodiment, the present invention relates to a self-referencing interferometric fiber optic sensor that is usable for measuring pressure.
2. Description of Related Art
Various types of sensor systems that utilize fiber optic technology are known. Such systems include systems in which a transducer mechanism is coupled to an optical fiber, as well as systems in which a transducer mechanism is incorporated into the optical fiber itself.
One example of a parameter that has been measured using a fiber optic sensor is fluid pressure. For example, according to one known arrangement, optical fibers are used to construct an interferometric system that monitors the position of a test mirror in order to provide an indication of pressure. The test mirror serves as a transducer mechanism and is movable in accordance with a pressure applied to one side of the mirror. In operation, light having a long coherence length is delivered from a light source (typically a laser) into a first optical fiber. The light travels through the optical fiber to a coupler. One portion of the light that is output from the coupler travels through a second optical fiber to a reference mirror, and then returns through the second optical fiber to the coupler. Another portion of the light output from the coupler travels through a third optical fiber to the test mirror and then returns through the third optical fiber to the coupler. At the coupler, constructive and destructive interference occurs between the light reflected from the reference mirror and the light reflected from the test mirror, depending on the position of the test and reference mirrors relative to the coupler. The interference is detected by a photodetector, which receives the interference from the coupler through a fourth optical fiber. The position of the mirror is then determined by monitoring the occurrence of interference fringes, one fringe corresponding to one-half wavelength of movement of the test mirror. To obtain pressure, the displacement of the mirror is multiplied by the spring constant of the mounting fixture for the test mirror and divided by the area of the mirror.
The advantage of interferometric fiber optic sensor systems is that they are very precise. For example, in the pressure measurement system just described, displacement of the test mirror on the nanometer scale may be detected.
In practice, however, difficulty has been encountered in fully exploiting the potential of interferometric fiber optic sensor systems. The optical paths between the coupler and the test and reference mirrors are affected differently over time by such factors as temperature variations, vibrations, optical fiber bending, and variations in refractive indices. These variations reduce the precision that can be achieved, because they introduce optical path length differences that are not caused by movement of the test mirror.
Additionally, in the context of pressure measurement systems of the above-described type, another difficulty that has been encountered is that such systems only provide only relative position information. Monitoring interference fringes only provides information about the movement of the test mirror, and not about its absolute position. Therefore, information about a previous position of the test mirror is always required in order to provide information about the current position of the test mirror. This allows for the development of accumulated error.
Thus, what is needed is an interferometric fiber optic sensor system that is relatively insensitive to variations in temperature, vibrations, optical fiber bending, and variations in refractive indices between the test and reference paths. In the context of pressure sensor systems, what is also needed is a system that allows absolute measurements of the position of a test mirror serving as a transducer mechanism to be made.